All-vanadium sulfate acid redox flow battery system

ABSTRACT

All-vanadium sulfate redox flow battery systems have a catholyte and an anolyte comprising an aqueous supporting solution including chloride ions and phosphate ions. The aqueous supporting solution stabilizes and increases the solubility of vanadium species in the electrolyte, allowing an increased vanadium concentration over a desired operating temperature range. According to one example, the chloride ions are provided by MgCl2, and the phosphate ions are provided by (NH4)2HPO4.

CROSS-REFERENCE TO RELATED APPLICATION

This claims the benefit of the earlier filing date of U.S. ProvisionalApplication No. 62/060,438, filed Oct. 6, 2014, which is incorporated byreference in its entirety herein.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under DE-AC0576RL01830awarded by the U.S. Department of Energy. The government has certainrights in the invention.

FIELD

This invention concerns embodiments of an all-vanadium sulfate acidelectrolyte comprising chloride ions and phosphate ions for use in aredox-flow battery system.

BACKGROUND

A redox flow battery (RFB) stores electrical energy in reduced andoxidized species dissolved in two separate electrolyte solutions. Theanolyte and the catholyte circulate through a cell electrode separatedby a membrane or separator. Redox flow batteries are advantageous forenergy storage because they are capable of tolerating fluctuating powersupplies, repetitive charge/discharge cycles at maximum rates,overcharging, overdischarging, and because cycling can be initiated atany state of charge.

However, among the many redox couples upon which redox flow batteriesare based, a number of disadvantages exist. For example, many systemsutilize redox species that are unstable, are highly oxidative, aredifficult to reduce or oxidize, precipitate out of solution, and/orgenerate volatile gases. One of the main challenges confronting RFBsystems is the intrinsically low energy density compared with otherreversible energy storage systems such as lithium-ion batteries. Withthe voltage limitation of the aqueous systems, this issue is typicallytackled by increasing the active species concentration in theelectrolyte. However, the active species concentration is limited by thesolubility and the stability of the active redox ions in the electrolytesolutions. Therefore, a need exists for RFB systems having a greaterenergy density.

SUMMARY

Embodiments of an all-vanadium sulfate acid flow battery system aredisclosed. The system includes an anolyte comprising V²⁺ and V³⁺ in anaqueous supporting solution, and a catholyte comprising V⁴⁺ and V⁵⁺ inan aqueous supporting solution. The aqueous supporting solution for eachof the anolyte and the catholyte includes sulfate ions, protons,chloride ions, and phosphate ions. The chloride ions may be provided byan inorganic chloride salt. The phosphate ions may be provided by aninorganic phosphate salt. In any or all of the above embodiments, theaqueous supporting solution may further include magnesium ions, ammoniumions, or magnesium ions and ammonium ions. In any or all of the aboveembodiments, [V] may be ≥1.0 M in the anolyte and [V] may be ≥1.0 M inthe catholyte.

In any or all of the above embodiments, the anolyte and catholyte maycomprise, consist essentially of, or consist of a solution prepared bycombining H₂O, VOSO₄, H₂SO₄, MgCl₂, and ammonium phosphate to provide1.0-2.5 M VOSO₄, 3.5 M H₂SO₄, 0.025-0.25 M MgCl₂, and 0.05-0.5 Mammonium phosphate. In some embodiments, the ammonium phosphate is(NH₄)₂HPO₄.

In any or all of the above embodiments, the battery system may have aMg:V molar ratio within a range of 1:100 to 1:1, a Cl:V molar ratiowithin a range of 1:50 to 1:2, a phosphate:V molar ratio within a rangeof 1:50 to 1:2, a NH₄+:V molar ratio within a range of 1:50 to 1:3, aCl:phosphate molar ratio within a range of 10:1 to 1:10, a NH₄+:Mg molarratio within a range of 60:1 to 1:5, or any combination thereof. In anyor all of the above embodiments, the sulfate concentration may be 4.5-6M.

In any or all of the above embodiments, the aqueous supporting solutionmay include H₂O, H₂SO₄, MgCl₂, and ammonium phosphate. In someembodiments, the aqueous supporting solution comprises 0.025-0.25 MMgCl₂ and 0.05-0.5 M ammonium phosphate.

In any or all of the above embodiments, the anolyte may comprise 1.0-2.5M vanadium as V²⁺ and V³⁺, 0.025-0.5 M magnesium ions, 0.05-0.5 Mchloride ions, 0.05-1.5 M ammonium ions, and 0.05-0.5 M phosphate ions,and the catholyte may comprise 1.0-2.5 M vanadium as V⁴⁺ and V⁵⁺,0.025-0.25 M magnesium ions, 0.0.5-0.5 M chloride ions, 0.05-0.1.5 Mammonium ions, and 0.05-0.5 M phosphate ions. In some embodiments, theanolyte and catholyte independently comprise 0.5-0.1 M MgCl₂ and 0.1-0.2M (NH₄)₂HPO₄.

In any or all of the above embodiments, the anolyte and catholyte maycomprise, consist essentially of, or consist of a solution prepared bycombining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide1.0-2.5 M VOSO₄-3.5 M H₂SO₄, 0.025-0.25 M MgCl₂, and 0.05-0.5 M ammoniumphosphate. In some embodiments, the ammonium phosphate is (NH₄)₂HPO₄.

In any or all of the above embodiments, the system may have an operatingcell temperature within a range of −5° C. to 50° C. In any or all of theabove embodiments, the system may further include an anode, a cathode,and a separator or membrane separating the anolyte and the catholyte. Insome embodiments, the anode and cathode are graphite or carbon-basedelectrodes.

Embodiments of an anolyte and catholyte system for use in an allvanadium sulfate acid redox flow battery system include an aqueousanolyte comprising V²⁺, V³⁺, sulfate, magnesium ions, ammonium ions,chloride ions, and phosphate ions; and an aqueous catholyte comprisingV⁴⁺, V⁵⁺, sulfate, magnesium ions, ammonium ions, chloride ions, andphosphate ions. In some embodiments, the anolyte and the catholyteindependently comprise 0.025-0.25 M MgCl₂. In any or all of the aboveembodiments, the anolyte and the catholyte may independently comprise0.05-0.5 M ammonium phosphate. In any or all of the above embodiments,the anolyte and catholyte system may have a Cl:V molar ratio within arange of 1:50 to 1:2 and a phosphate:V molar ratio within a range of1:50 to 1:2.

The foregoing and other objects, features, and advantages of theinvention will become more apparent from the following detaileddescription, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cyclic voltammogram on a carbon electrode of electrolytesolutions containing 2 M VOSO₄ and 3.5 M H₂SO₄ with and without addedcomponents. The scan was carried out at room temperature at a scan rateof 50 mV/s.

FIG. 2 is a graph showing the charge capacity, discharge capacity,charge energy, and discharge energy of an embodiment of the all vanadiumsulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄ and 0.05 M MgCl₂ as afunction of the number of cycles at −5° C.

FIG. 3 is a graph showing the coulombic efficiency (CE), energyefficiency (EE), and voltage efficiency (VE) of an embodiment of the allvanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄ and 0.05 MMgCl₂ as a function of the number of cycles at −5° C.

FIG. 4 is a graph showing the charge capacity, discharge capacity,charge energy, and discharge energy of an embodiment of the all vanadiumsulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄ and 0.05 M MgCl₂ as afunction of the number of cycles at 50° C.

FIG. 5 is a graph showing the coulombic efficiency (CE), energyefficiency (EE), and voltage efficiency (VE) of an embodiment of the allvanadium sulfate acid electrolyte with 0.15 M (NH₄)₂HPO₄ and 0.05 MMgCl₂ as a function of the number of cycles at 50° C.

FIGS. 6A and 6B are molecular models showing optimized DFT (densityfunctional theory) configurations for [V₂O₃(H₂O)₇]⁴⁺ (FIG. 6A) and[VO₂Mg(H₂O)₉]³⁺ (FIG. 6B) solvates; H₂O and H₃O⁺ solvation molecules areshown in stick images.

FIG. 7 shows ⁵¹V NMR spectra of vanadate-based electrolytes with andwithout added components; the spectra were measured at 250K under 11Tesla.

DETAILED DESCRIPTION

Embodiments of an all-vanadium sulfate acid flow battery system aredisclosed. The system includes a liquid-phase anolyte comprising V²⁺ andV³⁺ and a liquid-phase catholyte comprising V⁴⁺ and V⁵⁺. The anolyte andcatholyte comprise sulfuric acid and a dual-component system to increasesolubility and/or stability of the vanadium species, wherein thecomponents comprise chloride ions and phosphate ions. The term“phosphate ions” includes PO₄ ³⁻, HPO₄ ²⁻, and H₂PO⁴⁻ ions, and allcombinations thereof.

Embodiments of the system include a higher concentration of vanadiumand/or have an increased operating temperature range compared tovanadium sulfate systems in the absence of the chloride ions andphosphate ions.

I. Definitions and Abbreviations

The following explanations of terms and abbreviations are provided tobetter describe the present disclosure and to guide those of ordinaryskill in the art in the practice of the present disclosure. As usedherein, “comprising” means “including” and the singular forms “a” or“an” or “the” include plural references unless the context clearlydictates otherwise. The term “or” refers to a single element of statedalternative elements or a combination of two or more elements, unlessthe context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms usedherein have the same meaning as commonly understood to one of ordinaryskill in the art to which this disclosure belongs. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of the present disclosure, suitable methods andmaterials are described below. The materials, methods, and examples areillustrative only and not intended to be limiting. Other features of thedisclosure are apparent from the following detailed description and theclaims.

Unless otherwise indicated, all numbers expressing quantities ofcomponents, percentages, temperatures, times, and so forth, as used inthe specification or claims are to be understood as being modified bythe term “about.” Accordingly, unless otherwise indicated, implicitly orexplicitly, the numerical parameters set forth are approximations thatmay depend on the desired properties sought and/or limits of detectionunder standard test conditions/methods as known to those persons ofordinary skill in the art. When directly and explicitly distinguishingembodiments from discussed prior art, the embodiment numbers are notapproximates unless the word “about” is recited.

In order to facilitate review of the various embodiments of thedisclosure, the following explanations of specific terms are provided:

Capacity: The capacity of a battery is the amount of electrical charge abattery can deliver. The capacity is typically expressed in units ofmAh, or Ah, and indicates the maximum charge a battery can produce overa period of one hour. The specific capacity is measured with respect toeither volume or weight of the active materials, which leads to theconcept of specific volumetric capacity and specific gravimetriccapacity, respectively.

Cell: As used herein, a cell refers to an electrochemical device usedfor generating a voltage or current from a chemical reaction, or thereverse in which a chemical reaction is induced by a current. Examplesinclude voltaic cells, electrolytic cells, redox flow cells, and fuelcells, among others. Multiple single cells can form a cell assembly,often termed a stack. A battery includes one or more cells, or even oneor more stacks. The terms “cell” and “battery” are used interchangeablywhen referring to a battery containing only one cell.

Coulombic efficiency (CE): The efficiency with which charges aretransferred in a system facilitating an electrochemical reaction. CE maybe defined as the amount of charge exiting the battery during thedischarge cycle divided by the amount of charge entering the batteryduring the charging cycle.

Electrochemically active element: An element that is capable of formingredox pairs between different oxidation and reduction states, i.e.,ionic species with differing oxidation states. In a flow battery, anelectrochemically active element refers to the chemical species thatsignificantly participate in the redox reaction during the charge anddischarge processes contributing to the energy conversions thatultimately enable the battery to deliver/store energy. As used herein,the term “electrochemically active element” refers to an element thatconstitutes at least 5% of the redox active materials participating inredox reactions during battery cycling after initial charging.

Electrolyte: A substance containing free ions that behaves as anionically conductive medium. In a redox flow battery, some of the freeions are electrochemically active elements. An electrolyte in contactwith the anode, or negative half-cell, may be referred to as an anolyte,and an electrolyte in contact with the cathode, or positive half-cell,may be referred to as a catholyte. With respect to vanadium sulfate acidredox flow battery systems, the electrolyte conventionally refers tovanadium species in an aqueous sulfuric acid solution. As used herein,the terms “anolyte” and “catholyte” refer to vanadium species in anaqueous “supporting solution.” The supporting solution, or supportingelectrolyte, is an aqueous solution comprising sulfate ions, chlorideions, phosphate ions, protons, and other counterions introduced throughadded components that are not redox active. The anolyte and catholyteare often referred to as the negative electrolyte and positiveelectrolyte, respectively, and these terms can be used interchangeably.

Energy efficiency (EE): The product of coulombic efficiency and voltageefficiency. EE=CE×VE.

Half-cell: An electrochemical cell includes two half-cells. Eachhalf-cell comprises an electrode and an electrolyte. A redox flowbattery has a positive half-cell in which ions are oxidized, and anegative half-cell in which ions are reduced during charge. Oppositereactions happen during discharge. In an all vanadium redox flowbattery, VO²⁺ ions in the positive half-cell are oxidized to VO₂+ ions(V⁴⁺ oxidized to V⁵⁺), and V³⁺ ions in the negative half-cell arereduced to V²⁺ ions during charge.

Proton: Hydrogen ions (H⁺) and water-solvated hydrogen ions, e.g., H₃O⁺,[H₅O₂]⁺, [H₉O₄]⁺.

Voltage efficiency: The voltage produced by the battery whiledischarging divided by the charging voltage

II. All-Vanadium Sulfate Acid Redox Flow Battery System

Redox flow batteries (RFBs) can provide electrical energy converted fromchemical energy continuously, and are promising systems for energystorage to integrate renewable energies (e.g., solar and/or wind energy)into electrical supply grids. A RFB system comprises a positivehalf-cell and a negative half-cell. The half-cells are separated by amembrane or separator, such as an ion-conductive membrane or separator.The positive half-cell contains a catholyte and the negative half-cellcontains an anolyte. The anolyte and catholyte are solutions comprisingelectrochemically active elements in different oxidation states. Theelectrochemically active elements in the catholyte and anolyte couple asredox pairs. During use, the catholyte and anolyte are continuouslycirculating through the positive and negative electrodes, respectively,where the redox reactions proceed providing the conversion betweenchemical energy and electrical energy or vice-versa. To complete thecircuit during use, positive and negative electrodes of a RFB areelectrically connected through current collectors with an external load.

Among various RFBs, the all-vanadium redox flow battery (VRFB) iscurrently considered one of the most promising candidates for grid scaleenergy storage. As those of ordinary skill in the art understand, theterm “all-vanadium” means that the major redox active materials (i.e.,at least 95%, at least 97%, or at least 99% of the redox activematerials) participating in redox reactions during battery cycling(charging/discharging) after initial charging are vanadium ion redoxpairs, i.e., V²⁺, V³⁺, V⁴⁺⁻ (VO²⁺), V⁵⁺ (VO₂₊). Other redox pairs mayparticipate during initial charging of the redox flow battery.

The VRFB has several advantages such as high energy efficiency, quickresponse time, long lifespan, low self-discharge, no crossover issues,and low maintenance cost. However, the disadvantages of low energydensity and poor stability and solubility of flow battery electrolytesolutions limit its applications.

Embodiments of the disclosed all-vanadium sulfate acid redox flowbattery system have an anolyte comprising V²⁺ and V³⁺ in an aqueoussupporting solution and a catholyte comprising V⁴⁺ and V⁵⁺ in an aqueoussupporting solution, wherein the aqueous supporting solution comprisessulfate ions, protons, and a dual-component system comprising chlorideions and phosphate ions to stabilize and increase the solubility of thevanadium species. Without wishing to be bound by a particular theory, itappears that the combination of sulfate ions, protons, chloride ions,and phosphate ions may act synergistically to stabilize and increasesolubility of the vanadium species. Suitable electrodes include graphiteand/or carbon based electrodes, e.g., graphite felt, graphene, carbonfelt, carbon foam electrodes, and so on. In some embodiments, thecathode and the anode are graphite electrodes. The system furtherincludes a separator or membrane, such as an ion-conducting separator ormembrane, separating the anolyte and the catholyte. The vanadium may beprovided, for example, by dissolving vanadium (IV) oxide sulfate hydrate(VOSO₄.xH₂O) in sulfuric acid. The system is operable, without anexternal heat management device, over an operating temperature range of−5° C. to 50° C. with vanadium concentrations up to 2.5 M, such as from1.0-2 M. The flow battery system may be operated as a closed system,which advantageously prevents rapid oxidation of V²⁺ and V³⁺ by air andminimizes electrolyte loss.

Traditional vanadium sulfate RFB systems previously have been limited toa maximum concentration of 1.5 M vanadium in the absence of chloride,e.g., 1.5 M VOSO₄ and 3.5 M H₂SO₄, over an operating temperature rangeof 10° C. to 35° C. At greater concentrations, V⁵⁺ species precipitateat elevated temperatures (e.g., at 50° C.) and V⁴⁺ species precipitateat lower temperatures (e.g., less than 0° C.). For example, atconcentrations >1.7 M, V⁵⁺ has poor thermal stability at temperatures40° C. V²⁺ and V³⁺ ions also may precipitate at lower temperatures.

The disclosed supporting solutions include a dual-component system tomaintain electrolyte stability, thereby enabling increased vanadiumconcentration and thus the energy density of the system. In particular,the supporting solutions comprise a dual-component system that increasesthe solubility and stability of vanadium (IV) and vanadium (V) species.Effective components do not adversely react with the redox species,e.g., do not change the potential of redox reaction between V⁵⁺ and V⁴⁺,and do not react with sulfate. Effective components also areelectrochemically stable over the flow battery operation voltage window.Desirably, the added components are effective at low concentrationsrelative to the vanadium species and sulfuric acid; high added componentconcentrations may exacerbate vanadium species precipitation,particularly V³⁺, and/or form sulfate precipitates.

Embodiments of the disclosed supporting solutions comprise sulfate ions(e.g., provided by sulfuric acid), protons, and a dual-component systemcomprising chloride ions and phosphate ions. In one embodiment, theprotons are hydrogen ions (H⁺). In another embodiment, the protons aresolvated hydrogen ions, e.g., H₃O⁺, [H₅O₂]⁺, [H₉O₄]⁺, and combinationsthereof. In an independent embodiment, the protons are a combination ofH⁺, H₃O⁺, [H₄O₂]⁺, and/or [H₉O₄]⁺. The protons may be provided bysulfuric acid. The chloride ions may be provided by an inorganicchloride salt. As used herein, the term “inorganic chloride salt” refersto a salt composed of metal or ammonium cations and chloride anions. Thephosphate ions may be provided by an inorganic phosphate salt. The term“inorganic phosphate salt” refers to a salt composed of metal orammonium cations and phosphate ions (H₂PO₄ ⁻, HPO₄ ²⁻, PO₄ ³⁻, andcombinations thereof). The chloride ions and phosphate ions increase thestability and/or solubility of the vanadium species in the anolyte andcatholyte.

For a number of years following the development of all-vanadium redoxbattery systems, those of ordinary skill in the art determined andstated, that is, the conventional wisdom and understanding was, that HClcould not be present in supporting solutions for all-vanadium redoxbattery systems and particularly in supporting solutions for thecatholyte. For example, those of ordinary skill in the art believed thatV⁵⁺ is unstable in HCl solutions. Further, HCl is corrosive and canproduce dangerous HCl vapors and Cl₂ vapors if overcharging, especiallyat higher temperatures. The inventors however, unexpectedly discoveredthat, in some cases, very low concentrations of chloride ion, e.g.,0.1-0.2 M, can be advantageous in stabilizing the electrolyte. Theinventors also unexpectedly discovered that very low concentrations ofchloride ion stabilize the anolyte and catholyte at higher temperatures,e.g., above 35° C. In a preferred embodiment of the invention, chlorideions in the disclosed supporting solutions are provided by an inorganicchloride salt. The use of an inorganic chloride salt makes thesupporting solutions less corrosive than supporting solutions comprisingHCl in addition to H₂SO₄, and also advantageously reduces systemcorrosion and HCl vapors within the system. Inorganic chloride salts arepreferable to organic chloride salts, which can reduce V⁵⁺ to V⁴⁺.

Magnesium chloride, MgCl₂, was surprisingly found to be a particularlyeffective inorganic chloride salt. Without wishing to be bound by aparticular theory, Mg²⁺ ions may further augment stability of thevanadium species. Although calcium chloride may be a more effectiveanti-freezing agent with respect to vanadium, calcium can react withsulfate to form precipitated calcium sulfate. In some embodiments, thesupporting solution includes 0.05-0.1 M MgCl₂, thereby providing 0.1-0.2M chloride ions.

While inorganic chloride salts are effective at higher temperatureswithin the redox flow battery's operating temperature range, inorganicchloride salts may not sufficiently stabilize the vanadium species atlower temperatures, e.g., less than 10° C. Thus, it is beneficial toinclude a second added component that stabilizes the anolyte andcatholyte at low temperatures. Phosphate ions were found to stabilizethe electrolytes at low temperatures and increase the solubility ofvanadium (IV) species. In some embodiments, the supporting solutioncomprises 0.1-0.2 M phosphate ions. Phosphate ions may be provided by aninorganic phosphate salt. In some embodiments, phosphate ions areprovided by ammonium phosphate. As used herein, the term “ammoniumphosphate” refers to (NH₄)H₂PO₄, (NH₄)₂HPO₄, (NH₄)₃PO₄, and combinationsthereof. In certain examples, phosphate ions were provided by ammoniumhydrogen phosphate (ammonium phosphate dibasic), (NH₄)₂HPO₄.

Cyclic voltammetry showed that MgCl₂ and (NH₄)₂HPO₄ do not react withthe vanadium species (see, e.g., FIG. 1). MgCl₂ did not noticeablyaffect the scans. However, (NH₄)₂HPO₄ was found to have a catalyticeffect, providing higher peak currents of electrolyte with positivescanning from V⁴⁺ to V⁵⁺ and V⁵⁺ to V⁴⁺. Furthermore, the peak voltagefor V⁴⁺ to V⁵⁺ was shifted 75 mV, indicating that (NH₄)₂HPO₄ facilitatesthe oxidation. On the negative side, presence of (NH₄)₂HPO₄ shifted thepeak voltage 100 mV and produced peak currents that were wider andhigher, indicating that (NH₄)₂HPO₄ increases redox activity in theanolyte.

When the anolyte and catholyte include the dual-component systemcomprising chloride ions and phosphate ions, the anolyte and catholytemay independently have a total vanadium concentration (i.e., V²⁺/V³⁺,V⁴⁺/V⁵⁺) greater than 1.0 M, such as within the range of 1.0-2.5 M,1.5-2.5 M, or 1.5-2.0 M. Increasing the vanadium concentrationcorrespondingly increases the energy capacity of the system. In someexamples, the total vanadium concentration in each of the anolyte andthe catholyte is 2 M, an increase of 33% compared to vanadium-sulfatesystems without the disclosed dual-component system comprising chlorideions and phosphate ions, thereby providing an increased energy capacityof 33%. A vanadium-sulfate redox flow battery system without thedisclosed dual-component system and having a total vanadiumconcentration of 1.5M has an energy density of 20.1 Wh/L, based ontheoretical voltage and 80% of state of charge (SOC). In contrast, avanadium-sulfate redox flow battery system with the discloseddual-component system and having a total vanadium concentration of 2Mhas an energy density of 26.8 Wh/L, based on theoretical voltage and 80%SOC.

The cell reactions during battery charge/discharge cycling, afterinitial charging, are as shown below for a standard vanadium sulfateredox flow battery and a redox flow battery as disclosed herein:

At the negative electrode of a vanadium sulfate RFB:V³⁺ +e ⁻

V²⁺ E°=−0.26 V

At the positive electrode of a vanadium sulfate RFB:VO²⁺+2H⁺ +e ⁻

VO₂₊+H₂O E°=1.00 V

Embodiments of the disclosed all vanadium sulfate redox flow batterysystems include an anolyte aqueous supporting solution and a catholyteaqueous supporting solution that comprise sulfate, chloride ions, andphosphate ions. The anolyte aqueous supporting solution and catholyteaqueous supporting solution may further comprise magnesium ions,ammonium ions, or magnesium and ammonium ions. The anolyte furthercomprises V²⁺ and V³⁺ under battery charge/discharge conditions. Thecatholyte further comprises V⁴⁺ and V⁵⁺ under batterydischarge/discharge conditions.

In some embodiments, the anolyte aqueous supporting solution andcatholyte aqueous supporting solution independently comprise 0.05-0.5 M,such as 0.1-0.2 M, chloride ions provided by an inorganic chloride salt.The inorganic chloride salt may be MgCl₂. In some embodiments, theanolyte aqueous supporting solution and catholyte aqueous supportingsolution independently comprise 0.05-0.5 M, such as 0.1-0.2 M, phosphateions. Phosphate ions may be provided by an inorganic phosphate salt. Insome embodiments, the phosphate salt is ammonium phosphate, e.g.,ammonium phosphate dibasic.

In some embodiments, the anolyte and catholyte supporting solutionsindependently comprise, or consist essentially of, H₂O, H₂SO₄, magnesiumions, chloride ions, ammonium ions, and phosphate ions. In anindependent embodiment, the anolyte and catholyte independentlycomprise, or consist essentially of, H₂O, VOSO₄, H₂SO₄, MgCl₂, andammonium phosphate. As used herein, “consists essentially of” means thatthe electrolyte includes no other components that materially affectbattery performance. The electrolyte may include components that do notmaterially affect battery performance during charging/discharging, forexample, non-electrochemically active species such as alkali metalcations. The anolyte and catholyte may independently comprise, orconsist essentially of, H₂O, VOSO₄, H₂SO₄, 0.025-0.25 M MgCl₂, and0.05-0.5 M ammonium phosphate. In some embodiments, the anolyte andcatholyte may independently comprise, or consist essentially of, H₂O,VOSO₄, H₂SO₄, 0.05-0.1 M MgCl₂, and 0.1-0.2 M ammonium phosphate. In anindependent embodiment, the anolyte and catholyte consist of H₂O, VOSO₄,H₂SO₄, MgCl₂, and ammonium phosphate. The anolyte and catholyte mayindependently consist of H₂O, VOSO₄, H₂SO₄, 0.025-0.25 M MgCl₂, and0.05-0.5 M ammonium phosphate. In some embodiments the anolyte andcatholyte independently consist of H₂O, VOSO₄, H₂SO₄, 0.05-0.1 M MgCl₂,and 0.1-0.2 M ammonium phosphate. The ammonium phosphate may be(NH₄)₂HPO₄.

In some embodiments, during battery charge/discharge, the anolyte has atotal [V]≥1.0 M as V²⁺ and V³⁺, and the catholyte has a total [V]≥1.0 Mas V⁴⁺ (e.g., VO²⁺) and V⁵⁺ (e.g., VO₂ ⁺). The anolyte and catholyte mayindependently have a total concentration of 1.0-2.5 M vanadium or1.5-2.5 M vanadium, such as a concentration of 2 M vanadium.

In one embodiment, the anolyte and catholyte independently have a Cl:Vmolar ratio within a range of 1:50 to 1:2, such as from 1:25 to 1:4, orfrom 1:20 to 1:10. In some examples, the Cl:V molar ratio was 1:20.

In an independent embodiment, the anolyte and catholyte independentlyhave a phosphate:V molar ratio within a range of 1:50 to 1:2, such asfrom 1:25 to 1:4, or from 1:20 to 1:10. In some examples, thephosphate:V molar ratio was 1:13.

In an independent embodiment, the anolyte and catholyte independentlyhave a Mg:V molar ratio within a range of 1:100 to 1:1, such as 1:50 to1:8, or from 1:40 to 1:20. In some examples, the Mg:V molar ratio was1:40.

In an independent embodiment, the anolyte and catholyte independentlyhave a NH₄ ⁺:V molar ratio within a range of 1:50 to 1:3, such as from1:25 to 1:1.3, or 1:10 to 1:5. In some examples, the NH₄+:V molar ratiowas 1:6.7.

In an independent embodiment, the anolyte and catholyte independentlyhave a Cl:SO₄ molar ratio within a range of 1:120 to 1:9, such as from1:60 to 1:25, or 1:55 to 1:35. In some examples, the Cl:SO₄ molar ratiowas 1:55.

In an independent embodiment, the anolyte and catholyte independentlyhave a phosphate:SO₄ molar ratio within a range of 1:120 to 1:9, such asfrom 1:60 to 1:25, or 1:50 to 1:25. In some examples, the phosphate:SO₄molar ratio was 1:37 or 1:27.

In an independent embodiment, the anolyte and catholyte independentlyhave a Cl:phosphate molar ratio within a range of 10:1 to 1:10, such asfrom 5:1 to 1:5, from 2:1 to 1:2, or from 1:1 to 1:2. In some examples,the Cl:phosphate molar ratio was 1:1.5.

In an independent embodiment, the anolyte and catholyte independentlyhave a NH₄ ⁺:Mg molar ratio within a range of 60:1 to 1:5, such as from12:1 to 1:1, or from 8:1 to 4:1. In some examples, the NH₄ ⁺:Mg molarratio was 6:1 or 8:1.

In an independent embodiment, the anolyte and catholyte independentlyhave a Mg:Cl molar ratio of 1:2.

In an independent embodiment, the anolyte and catholyte independentlyhave a NH₄+:phosphate molar ratio within a range of 3:1 to 1:1, such asfrom 2.5:1 to 1.5:1. In some examples, the NH₄+:phosphate molar ratiowas 2:1.

In an independent embodiment, the anolyte and catholyte independentlycomprise, consist essentially of, or consist of a solution prepared bycombining H₂O, VOSO₄—H₂SO₄, a magnesium ion source, a chloride ionsource, an ammonium ion source, and a phosphate ion source to provide1.0-2.5 M vanadium, 4.5-6 M SO₄ ²⁻, 0.025-0.25 M Mg²⁺, 0.05-0.5 M Cl⁻,0.05-1.5 M NH₄ ⁺, and 0.05-0.5 M phosphate. The anolyte and catholytemay independently comprise, consist essentially of, or consist of asolution prepared by combining H₂O, VOSO₄—H₂SO₄, a magnesium ion source,a chloride ion source, an ammonium ion source, and a phosphate ionsource to provide 1.0-2.5 M vanadium, 4.5-6 M SO₄ ²⁻, 0.05-0.1 M Mg²⁺,0.1-0.2 M Cl⁻, 0.1-0.6 M NH₄ ⁺, and 0.1-0.2 M phosphate. Suitable ionsources may include, but are not limited to, MgCl₂, (NH₄)₂HPO₄,(NH₄)H₂PO₄, (NH₄)₃PO₄, Mg(OH)₂, HCl, NH₄OH, H₃PO₄, NH₄Cl, MgHPO₄,Mg(H₂PO₄)₂, Mg₃(PO₄)₂, and NH₄MgPO₄.

In an independent embodiment, the anolyte and catholyte independentlycomprise, consist essentially of, or consist of a solution prepared bycombining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide1.0-2.5 M vanadium, 4.5-6 M SO₄ ²⁻, 0.025-0.25 M MgCl₂, and 0.05-0.5 Mammonium phosphate. The anolyte and catholyte may independentlycomprise, consist essentially of, or consist of a solution prepared bycombining H₂O, VOSO₄—H₂SO₄, MgCl₂, and ammonium phosphate to provide1.0-2.5 M vanadium, 4.5-6 M SO₄ ²⁻, 0.05-0.1 M MgCl₂, and 0.1-0.2 Mammonium phosphate or 0.15-2 M ammonium phosphate. The ammoniumphosphate may be (NH₄)₂HPO₄. In some examples, the anolyte and catholytecomprise 2 M vanadium, 5.5 M SO₄ ²⁻, 0.05 M MgCl₂, and 0.15 M(NH₄)₂HPO₄.

Embodiments of the disclosed all vanadium sulfate acid redox flowbattery systems are operable over a temperature range from −5° C. to 50°C. and a current density range from 1 mA/cm² to 1000 mA/cm². In someembodiments, the disclosed systems are operable over a temperature rangefrom −5° C. to 50° C. with a coulombic efficiency >90%, such as >95%or >97% at a current density from 10 mA/cm² to 320 mA/cm². The disclosedsystems have an energy efficiency and a voltage efficiency >75%, >80%,or >85% over the operating temperature range at a current density from10 mA/cm² to 320 mA/cm². Coulombic, energy, and/or voltage efficiencymay depend at least in part on other parameters, such as flow field,pump rate, etc.

At −5° C., embodiments of the disclosed all vanadium sulfate redox flowbattery systems including 2 M VOSO₄, 3.5 M H₂SO₄, 0.1-0.2 M Cl⁻ providedby an inorganic chloride salt, and 0.1-0.2 M phosphate ions have acharge capacity and discharge capacity greater than 1.2 Ah, a chargeenergy greater than 2 Wh, and a discharge energy greater than 1.6 Whover at least 75 cycles. In such embodiments, the voltage efficiency andenergy efficiency is greater than 80%, with a coulombic efficiencygreater than 95%. At 50° C., the same systems have a charge capacity anddischarge capacity from 1.8-2.2 Ah, a charge energy from 2.9-3.2 Wh, anda discharge energy from 2.7-3 Wh over at least 25 cycles. The voltageand energy efficiencies are greater than 85%, with a coulombicefficiency greater than 95%. Over the operating temperature range of −5°C. to 50° C. and a current density range from 1 mA/cm² to 1000 mA/cm²,the coulombic, energy, and voltage efficiencies of the disclosed systemsremain substantially constant (i.e., varying by less than 5%) over atleast 25 cycles, at least 50 cycles, at least 70 cycles, or at least 80cycles (see, e.g., FIGS. 3 and 5). Charge and discharge capacities alsoremain substantially constant (see, e.g., FIGS. 2 and 4).

In some examples where the electrolytes included 2 M vanadium, 5.5 Msulfate, 0.05 M MgCl₂, and 0.15 M (NH₄)₂HPO₄, the coulombic efficiencywas 97-99% and the energy efficiency was 80-89% over the temperaturerange from −5° C. to 50° C. at a current density of 50 mA/cm². Thecharge and discharge capacities were 1.3-2.2 Ah, the charge energy was2-3.2 Wh, and the discharge energy was 1.7-2.7 Wh.

III. Examples

Materials and Methods

Vanadium sulfate was purchased from Noah Technology. All other chemicalswere purchased from Sigma-Aldrich.

Solution Preparation and Stability Testing

The V⁴⁺ electrolyte solutions were prepared by dissolving VOSO₄.xH₂O insulfuric acid aqueous solutions. The electrolyte solutions containingV²⁺, V³⁺ and V⁵⁺ cations were prepared electrochemically by charging theV⁴⁺ solutions in a flow cell. The stability evaluations were carried outat a temperature range of −5 to 50° C. in a temperature-controlled bath.For the added component effect study, a known amount of each addedcomponent (i.e., chloride, phosphate) was added into the electrolytesolutions before starting the stability evaluations. All the stabilitytests were carried out statically without any stirring or shaking.During the evaluation, each sample was scanned once a day forprecipitation and solution color change. The concentrations ofelectrolyte solution were analyzed by inductively coupled plasma/atomicemission spectrometry (ICP/AES, Optima 7300DV, Perkin Elmer) techniquesafter appropriate dilution. Three emission lines were chosen for eachelement as a crosscheck for spectral interference. The calibrationstandards were matrix-matched in water.

Flow Cell Test of 2 M Vanadium Sulfate Acid Electrolyte Solution withMixture of (NH₄)₂HPO₄ and MgCl₂ as Added Components

The flow cell tests were conducted at 50, 25 and −5° C. respectively ina single flow cell with graphite plates as positive and negative currentcollectors. Graphite felt (“GFD5”, SGL Carbon group, Germany) wasemployed as both positive and negative electrodes, with geometrical areaof 10 cm² and heat treatment at 400° C. in air for 6 h. A Nafion® 115membrane (perfluorosulfonic acid/PTFE copolymer in the acid form) soakedin deionized water was used as a separator in the flow cell. Theelectrolyte solutions containing 2M V⁴⁺ ion with 0.15 M (NH₄)₂HPO₄ and0.05 M MgCl₂ in 5.5 M H₂SO₄ were employed and the flow rate was fixed at20 mL/min. Charging was conducted at 50 mA/cm² to 1.6 voltage (80% SOC)and discharged to 0.8 voltage with the same current density. To keep thecharge balance at positive and negative sides, the V⁵⁺ solution (ca.100% SOC) at positive side after the first charge process was replacedby the original V⁴⁺ solution with the same volume. Cycling test wasperformed to investigate capacity degradation and durability of theelectrolyte 2 M VOSO₄, 3.5 M H₂SO₄, 0.15 M (NH₄)₂HPO₄ and 0.05 M MgCl₂.Both the flow cell and electrolyte reservoirs are inside an environmentchamber (Thermal Product Solution, White Deer, Pa.) maintained at −5° C.during the test.

Example 1 Stability of All-Vanadium Sulfate Acid Electrolytes WithoutAdded Components

The highest concentration of all-vanadium sulfate acid electrolytesolution was 1.5 M over the operating temperature range of −5 to 50° C.as shown in Table 1. As shown in Table 1, species of V⁴⁺ and V⁵⁺ easilyprecipitated out from electrolyte solutions.

TABLE 1 Stability of V^(n+) sulfate acid solutions (Italicized datashows the conditions and times at which the indicated speciesprecipitated.) Time to V^(n+) Species V^(n+), M Total Sulfate, M T, ° C.precipitation V³⁺ 2.0 5.5 −5 Stable >10 days 2.0 5.5 25 Stable >10 days2.0 5.5 40 Stable >10 days 2.0 5.5 50 Stable >10 days V⁴⁺ 2.0 5.5 −5Stable <18 hours (VO²⁺) 2.0 5.5 25 Stable <95 hours 2.0 5.5 40Stable >10 days 2.0 5.5 50 Stable >10 days V⁵⁺ 2.0 5.5 −5 Stable >10days (VO₂ ⁺) 2.0 5.5 25 Stable >10 days 2.0 5.5 40 Stable <95 hours 2.05.5 50 Stable <18 hours 1.5 5.0 50 Stable >10 days

Example 2 Stability of All-Vanadium Sulfate Acid Electrolytes with AddedComponents

Inorganic salt anti-freezing components were chosen because organicsalts could react with V⁵⁺ species and reduce V⁵⁺ species to V⁴⁺ whichis unsuitable for the all-vanadium flow battery system. To determine thebest added components for the all-vanadium sulfate acid flow batterysystem, especially to improve the stability of V⁴⁺ and V⁵⁺ species,several inorganic salts were evaluated as shown in Table 2.

TABLE 2 Salts Chemical Name Formula Lowest Practical Temp. ° C. CalciumChloride CaCl₂ −29 Magnesium Chloride MgCl₂ −15 Potassium AcetateKC₂H₃O₂ −9 Lithium Chloride LiCl Ammonium Acetate NH₄C₂H₃O₂ SodiumChloride NaCl −9 Ammonium Sulfate (NH₄)₂SO₄ −7 Urea NH₂CONH₂ −7 AmmoniumPhosphate (NH₄)₂HPO₄ Dibasic Ammonium Phosphate NH₄H₂PO₄ MonobasicAmmonium Chloride NH₄Cl Sodium Sulfate Na₂SO₄

The added salts were evaluated at temperatures ranging from −5 to 50° C.The best salts were mixtures of ammonium phosphate dibasic and magnesiumchloride, which could stabilize all vanadium species of V³⁺, V⁴⁺ and V⁵⁺in a sulfate acid system. Although calcium chloride is a more efficientanti-freezing agent, calcium ions could react with sulfate acid to formundissolved calcium sulfate. Other anti-freezing agents or salts wereeither not strong enough to stabilize the V³⁺, V⁴⁺ and/or V⁵⁺ attemperature range of −5 to 50° C. or could react with sulfate acid toform undissolved salts. Ammonium phosphate dibasic and magnesiumchloride individually were not strong enough to stabilize all the fourvanadium species.

As shown in Table 1, V³⁺ is stable at temperatures ranging from −5 to50° C. However, any added salt could cause precipitation in highlyconcentrated electrolyte solutions. Therefore it is preferable toinclude minimal concentrations of added salts to avoid precipitation ofV³⁺. For chloride based salts, it has been shown that chloride ion canstabilize V⁵⁺ species in electrolyte solution (Li et al., AdvancedEnergy Materials 2011, 1:394-400). However, the stability studies showedthat chloride ion did not improve the solubility of V⁴⁺ species. It alsowas demonstrated that ammonium phosphate dibasic did not improve thesolubility and stability of V⁵⁺ electrolyte, but was a good salt forstabilizing V⁴⁺ species. The test results for all vanadium sulfate acidelectrolytes with different salts are shown in Table 3.

TABLE 3 Stability of V^(n+) sulfate acid solutions with salts(Italicized data shows the temperatures and times at which the indicatedspecies precipitated.) Time to V (M) SO₄ ²⁻(M) Composition Temperature °C. Salt precipitate 2.0 5.5 V⁵⁺ 50, 40, 0, −5, −10, −15 Blank <4 daysV⁴⁺ 50, 0, −5, −10, −15 <4 days V³⁺ 50, 0, −5, −10, −15 ≥48 days 2.0 5.5V⁵⁺ 50, 40, 0, −5, −10, −15 0.05M MgCl₂ V⁴⁺ 50, 0, −5, −10, −15 <1 dayV³⁺ 50, 0, −5, −10, −15 <5 days 2.0 5.5 V⁵⁺ 50, 40, 0, −5, −10 0.15M <2days V⁴⁺ 50, 0, −5, −10 (NH₄)₂HPO₄ <8 days V³⁺ 50, 0, −5, −10, −15 2.05.5 V⁵⁺ 50, 40, 25, 0, −5 0.1M ≥14 (50, 40° C.) days V⁴⁺ 50, 0, −5(NH₄)₂HPO₄ ≥12 (−5° C.) days V³⁺ 50, 0, −5, −10 0.1M MgCl₂ >60 days 2.05.5 V⁵⁺ 50, 40, 25, 0, −5 0.15M ≥21 (50, 40° C.) days V⁴⁺ 50, 0, −5(NH₄)₂HPO₄ ≥20 (−5° C.) days V³⁺ 50, 0, −5, −10, −15 0.05M MgCl₂ ≥41days (50° C.) 2.0 5.5 V⁵⁺ 50, 40, 25, 0, −5 C. 0.2M ≥21 days (50, 40°C.) V⁴⁺ 50, 0, −5, −10, −15 C. (NH₄)₂HPO₄ ≥22 days (0° C.) V³⁺ 50, 0,−5, −10 C. 0.05M MgCl₂ ≥21 (50° C.) days

The results demonstrated that a mixture of (NH₄)₂HPO₄ and MgCl₂ couldstabilize 2 M all-vanadium sulfate electrolytes. Mixtures of 0.15 or 0.2M (NH₄)₂HPO₄ and 0.05 M MgCl₂ were able to stabilize 2 M all-vanadiumsulfate electrolytes for more than 20 days at a temperature range from−5 to 50° C.

Example 3 Properties and Performance of All-Vanadium Sulfate AcidElectrolyte Solutions with (NH₄)₂HPO₄ and MgCl₂

Electrochemical properties of the all vanadium mixed acid electrolytewith and without added salts were investigated using the cyclicvoltammogram (CV) method. The CV scan of 2.0 M VOSO₄ and 3.5 M H₂SO₄electrolyte with and without 0.15 M (NH₄)₂HPO₄ and 0.05 M MgCl₂ areshown in FIG. 1.

As shown in FIG. 1, both ammonium phosphate dibasic and magnesiumchloride were unreactive with the vanadium species. The CV scans of 2 MVOSO₄, 3.5 M H₂SO₄ electrolyte with and without 0.05 M MgCl₂ were almostidentical, indicating that magnesium chloride does not react withvanadium species at all. As shown, however, ammonium phosphate dibasichad a catalytic effect on both positive and negative electrolytes. Whenpositive scanning from V⁴⁺ to V⁵⁺ and from V⁵⁺ to V⁴⁺, both the peakcurrents of electrolyte with ammonium phosphate dibasic were higher thanthat of electrolyte without ammonium phosphate dibasic. Also the peakvoltage was shifted 75 mV from 1.15V to 1.075V when scanning from V⁴⁺ toV⁵⁺, indicating ammonium phosphate dibasic can help V⁴⁺ to V⁵⁺ transfer.For negative side electrolytes, the peak voltage was shifted about 100mV from −0.835V to −0.735V and the peak currents were much wider andhigher than that without ammonium phosphate dibasic electrolytes. Theresults indicated that ammonium phosphate dibasic not only can improvethe stability and solubility of electrolytes, but also can increase theactivity of redox reaction of electrolytes.

The cell performance of the all vanadium sulfate acid electrolytesolution was evaluated in a single RFB cell. As shown in FIGS. 2 and 4,charge capacity, discharge capacity, charge energy, and discharge energyremained substantially constant for at least 80 cycles at −5° C. and forat least 25 cycles at 50° C.

As shown in FIGS. 3 and 5, respectively the columbic efficiency was 99%at −5° C. and 97% at 50° C. Energy efficiency was 80% at −5° C. and 89%at 50° C.; and voltage efficiency was about 80%. Voltage efficiency wassimilar to energy efficiency.

Example 4 Vanadate Solvate Structures

Understanding vanadate solvation structures facilitates rational designof redox flow battery electrolytes. Vanadium solvate structures wereanalyzed using density functional theory (DFT)-based calculations withthe NWchem 6.1 package (developed and maintained by the EnvironmentalMolecular Science Laboratory, at Pacific Northwest National Laboratory,Richland, Wash.; Valiev et al., Comput Phys. Commun. 2010, 181:1477).All of the calculations were done at the B3LYP theory level withdispersion correction (D3) using the 6-31G** (all-electron valencedouble zeta with polarization function) Gaussian-type basis set withoutany geometrical constraints. To capture the solvent ensemble effect,COSMO (an implicit solvent model) was employed with dielectric constant(ε) of 29.8 to represent the 5M sulfuric acid solution. Initially, theV⁵⁺ based solvates with water molecules alone were analyzed with COSMOmodel. For the global energy minimum structures, explicit solventmolecules (both H₂O and H₃O⁺) were evenly spread over the V⁵⁺ basedsolvates and subsequently treated with COSMO based solvation model (FIG.6A). This cluster model approach with layers of explicit and implicitsolvent molecules represents the highly acidic environment within theelectrolyte system.

The vanadate solvate structure (FIG. 6A) revealed that the watermolecule bonded with metal cation, i.e., [V₂O₃(H₂O)₇]⁴⁺, is prone tolosing a proton due to its electrostatic interaction with the metalcenter even under highly acidic conditions (represented by H₃O⁺ ions).The relatively higher electronegativity of the vanadate cation leads totransfer of electron density from the molecular orbital of the watermolecules to the empty orbitals of the metal cation. This chargetransfer weakens the O—H bond in the coordinated water molecules andmakes it more acidic, leading to proton loss and subsequentlyhydroxo-species which are prone to hydrolysis-based polymerizationleading to V₂O₅ precipitation. Although the proton loss can becontrolled by temperature and pH of the solvent system, the highercharge cations (Z≥4+) could still be susceptible to proton loss. Henceit is advantageous to counter the initial proton loss through additionof suitable added components, which can disrupt the hydrolysis-basedpolymerization by coordinating directly with the vanadate solvatestructure.

A Mg²⁺-based added component was selected because its lowerelectronegativity can inhibit the proton loss from the coordinated watermolecules and because it could also directly coordinate with thevanadate molecule through a bridging oxygen configuration. DFT analysisrevealed a lower formation energy (˜0.30 eV) for the Mg²⁺-coordinatedvanadate solvate structure (FIG. 6B) corroborating the advantages of theMg²⁺-based added component. Guided by this computational analysis,electrolytes were prepared with a high concentration (≥2M) of vanadateand a low concentration (0.1 M) of MgCl₂ as an added component system.The ⁵¹V NMR analysis of pristine and added component vanadate-basedelectrolytes is shown in FIG. 7. The added component-based electrolytereveals the formation of new species indicated by a low-field resonance(peak B ˜−592 ppm) relative to the pristine vanadate electrolyte (peak A˜−577 ppm), which corroborated the DFT-based prediction.

To further validate the identity of this new species under addedcomponent conditions, the ⁵¹V NMR chemical shift was calculated for[V₂O₃(H₂O)₇]⁴⁺ and [VO₂Mg(H₂O)₉]³⁺ solvates as −562 and −584 ppm,respectively. The DFT derived chemical shifts are in good agreement withthe observed chemical shifts of peak A and peak B, and confirm theformation of a Mg²⁺-coordinated vanadate solvate structure. Such Mg²⁺coordination can effectively disrupt the hydrolysis-based networkformation between successive vanadate molecules and thereby suppress theV₂O₅ precipitation.

Additional subject matter is disclosed in U.S. Pat. No. 8,628,880, whichis incorporated by reference in its entirety herein.

In view of the many possible embodiments to which the principles of thedisclosed invention may be applied, it should be recognized that theillustrated embodiments are only preferred examples of the invention andshould not be taken as limiting the scope of the invention. Rather, thescope of the invention is defined by the following claims. We thereforeclaim as our invention all that comes within the scope and spirit ofthese claims.

We claim:
 1. An all-vanadium sulfate acid redox flow battery systemcomprising: an anolyte comprising V²⁺ and V³⁺ in an aqueous supportingsolution, the anolyte having a total V²⁺/V³⁺ concentration within arange of 1.5-2.0 M; and a catholyte comprising V⁴⁺ and V⁵⁺ in an aqueoussupporting solution, the catholyte having a total V⁴⁺/V⁵⁺ concentrationwithin a range of 1.5-2.0 M; the aqueous supporting solution for each ofthe anolyte and the catholyte comprising 4.5-6 M sulfate ions, protons,0.05-0.1 M magnesium ions, 0.2-0.4 M ammonium ions, 0.1-0.2 M chlorideions, and 0.1-0.2 M phosphate ions, wherein the system is operable overa temperature range of from −5° C. to 50° C.
 2. The battery system ofclaim 1, wherein: the chloride ions are provided by MgCl₂; and thephosphate ions are provided by ammonium phosphate.
 3. The battery systemof claim 1, wherein the aqueous supporting solution comprises H₂O, H₂SO₄which provides protons and sulfate ions, MgCl₂ which provides magnesiumand chloride ions, and ammonium phosphate which provides ammonium andphosphate ions.
 4. The battery system of claim 1, wherein the anolyteand catholyte independently comprise a solution prepared by combiningH₂O, VOSO₄, H₂SO₄, MgCl₂, and ammonium phosphate to provide a solutioncomprising: 1.5-2.0 M vanadium; 4.5-6 M sulfate ions; 0.05-0.1 Mmagnesium ions; 0.1-0.2 M chloride ions; 0.1-0.6 M ammonium ions; and0.1-0.2 M phosphate ions.
 5. The battery system of claim 1, wherein theanolyte and catholyte independently consist essentially of a solutionprepared by combining water, VOSO₄, H₂SO₄, MgCl₂, and ammonium phosphateto provide: 1.5-2.0 M vanadium; 4.5-6 M sulfate ions; 0.05-0.1 Mmagnesium ions; 0.1-0.2 M chloride ions; 0.1-0.6 M ammonium ions; and0.1-0.2 M phosphate ions.
 6. An anolyte and catholyte system for use inan all vanadium sulfate acid redox flow battery system comprising: anaqueous anolyte comprising V²⁺, V³⁺, sulfate, 0.05-0.1 M magnesium ionsand 0.1-0.2 M chloride ions provided by MgCl₂, and 0.1-0.6 M ammoniumions and 0.1-0.2 M phosphate ions provided by ammonium phosphate; and anaqueous catholyte comprising V⁴⁺, V⁵⁺, sulfate, 0.05-0.1 M magnesiumions and 0.1-0.2 M chloride ions provided by MgCl₂, and 0.1-0.6 Mammonium ions and 0.1-0.2 M phosphate ions provided by ammoniumphosphate.
 7. The anolyte and catholyte system of claim 6, having a Cl:Vmolar ratio within a range of 1:50 to 1:2 and a phosphate:V molar ratiowithin a range of 1:50 to 1:2.
 8. The battery system of claim 1, whereinthe aqueous supporting solution for each of the anolyte and thecatholyte comprises: 0.05-0.1 M magnesium ions and 0.10-0.2 M chlorideions provided by MgCl₂; and 0.2-0.4 M ammonium ions and 0.1-0.2 Mphosphate ions provided by (NH₄)₂HPO₄.
 9. The battery system of claim 1,further comprising: an anode; a cathode; and a separator separating theanolyte and the catholyte.